INVISIBLE FORCE
LEADER NOTES
Photo credit: NASA/JPL
CHALLENGE: Design a setup so that when a steel ball rolls past a magnet, it changes direction and hits
a target that’s off to the side.
LEARNING GOALS: Science: Magnetism, inverse square law; NASA: Gravity-assisted travel;
Engineering: Design process
NASA CONNECTION: Many spacecraft fly close by a planet or moon to use its gravitational pull to change
speed and direction. This “gravity assist” also greatly reduces the fuel required to navigate a spacecraft.
GET READY AHEAD OF TIME
MATERIALS
• Magnets. The best magnets for this activity are the
silver, three-inch-long, oblong magnets sometimes called
“cow” or “snake-egg” magnets. If you use disc
magnets, kids may need to stack a few to produce a
strong magnetic field.
• Steel balls. Bike stores sell the 60-millimeter (quarterinch) ball bearings used in this activity. Slingshot ammo
is 80 millimeters (5/16 inch) and is also an option. Balls
larger than this build up so much momentum that they
don’t respond well to weak magnetic fields.
• Make a physical guide. In Step 2, kids use a rope to
guide the ball’s motion. Cut a 30-centimeter (12-inch)
length of clothesline per team. Wrap the ends with tape
to keep the rope from unraveling and forming a flare
that interferes with the ball traveling smoothly along the
rope.
• Get the videos. Go to pbskids.org/designsquad/links/
solarsystem. Download the Invisible Force, Victoria
Garcia, and NASA videos. Be prepared to project them.
If you’re unable to show videos, review the handout’s
overview and steps and tell kids about the NASA work
described in the overview and in Step 1.
(per team)
Invisible Force works well with teams of two.
• Paper cup (6- to 8-ounce)
• Strip of index card (2.5 x 12.5
centimeters [1 X 5 inches])
• 30-centimeter (12-inch) length of
flexible rope (e.g., clothesline)
• 1 steel ball (e.g., 60-millimeter
[quarter-inch] ball bearing)*
• 1 strong magnet**
• 1 target (e.g., “X” of tape on the
table or object to hit [e.g., a glass,
coin, cup, etc.])
• Tape (any kind)
* Available at bike stores. Slingshot
ammo from sporting-goods stores
works, too.
** Available at toy, dollar, craft, and
office supply stores.
• Photocopy. Copy the handout and performance
assessment rubric.
1
INTRODUCE THE CHALLENGE
(10 minutes)
Set the stage and give the NASA context
• Explain that gravity is a force present in all things and that planets and moons exert a gravitational
pull on spacecraft that pass close by.
Design Squad Nation™/© 2013 WGBH Educational Foundation.
• Spacecraft use small onboard rockets to make minor course corrections.
Big corrections require a lot of fuel, which is heavy to carry. Instead of
using rockets, NASA engineers use a planet’s or moon’s gravity to
increase a spacecraft’s speed and “steer” it by changing its direction.
This process is called a “gravity assist.”
• Tell kids that today they will do something similar to what NASA does, but
instead of using gravity to steer a ball, they’ll use a magnetic force. Then
show kids the Invisible Force video.
2
STEER THE BALL USING A PHYSICAL GUIDE (10 minutes)
• Get a ramp, piece of rope, and steel ball. Place a target right in front of
the ramp. Ask: What will happen when I roll this ball down the ramp? (The
ball will hit the target.)
• Next, place the target 15 centimeters (6 inches) to the right of the ramp.
Ask: What will happen now? (The ball will roll past the target.)
Kids roll a ball down a ramp
and use a ROPE to guide it
over to a target.
• Ask: How can I use this rope so the ball hits the target? (Curve the rope from the end of the ramp over
to the target.) Follow kids’ suggestions to hit the target with the ball.
• Emphasize that kids can change the ball’s speed—it goes faster when released from the top and slower
when released from the bottom. Release the ball from different points on the ramp to show that a ball
needs a certain amount of energy to make it to the target.
• Challenge kids to put the target in “wacky” locations around the ramp and use the rope to guide the
ball to the target. By trying different speeds and rope shapes, how creative can they get?
3
DISCUSS THE GAME’S KEY POINTS (5 minutes)
Stop kids’ play by asking them to stick the ball to the magnet and come over
to a demo area.
• Path shape. Without a guide, how will the ball travel once it leaves the
ramp? (Straight)
• Off-center positions. What were the “wackiest” target positions that your
ball hit?
• Speed and energy. How was the ball’s speed affected by where you
released it? Why is the ball’s speed important? (The greater the speed, the
greater the momentum. Momentum gives the ball energy to roll to the target.)
• Consistent results. It’s helpful to tape the ramp in place and release the
ball cleanly (e.g., hold it back with the tip of a sharp pencil).
4
STEER THE BALL USING A MAGNET
(5 minutes)
Set the ball on the magnet. It
will move to the pole—the
strongest part of the magnetic
field. Make a loop of duct tape
and stick it at 90 degrees
from the pole. Have kids tape
their magnets to the table so
the pole is in the same plane
as the rolling ball.
Tell kids that their new challenge is to replace the rope with a magnet that
will guide the ball. Set the rope aside. Tape down a strong magnet.
• Ask: How does this system compare to the one with the rope? (The magnet replaces the rope. Its
magnetic force will change the path of the ball. Otherwise, the setup is similar.)
• Ask: How big is the zone around the magnet that can affect the ball? (Small)
• Ask: How are the ball’s speed and distance from the magnet related? (A slowly moving ball has less
energy than a fast-moving one, and the magnet can capture it more easily. A faster ball can travel closer
to the magnet without crashing.)
• How is this small zone like the Goldilocks story? (Too close and you crash. Too far and you get lost. Just
right and the ball curves and hits the target.)
Design Squad Nation™/© 2013 WGBH Educational Foundation.
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BRAINSTORM AND DESIGN (5 minutes)
• How will you control how fast the “spacecraft” travels?
(Change the release point on the ramp.)
• What will you do if the spacecraft always crashes into the “planet?”
(Aim farther from the magnet.)
• What will you do if the spacecraft zips right by the planet?
(Aim closer to the magnet.)
BUILD, TEST, EVALUATE, AND REDESIGN (15 minutes)
If any of these issues come up, ask questions to get kids thinking about how
they might solve them.
• If kids can’t consistently launch a ball… Stabilize the ramp by taping it to
the table. Also tape the ramp to the cup.
• If kids can’t find the “zone” around the magnet… Tell them that a
magnet’s strength drops off quickly with distance. Also have them
experiment with the ball’s speed.
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Kids roll a ball down a ramp
and use a MAGNET to guide
it over to a target.
DISCUSS WHAT HAPPENED
(10 minutes)
Emphasize key elements in today’s challenge by asking:
• Engineering: How did testing help you decide how to change to your setup? (Answers will vary.)
• Science: Why did your ball change direction? What did you have to do to get it to change direction?
(The ball passed through a magnetic field. To change the path, the ball has to pass close enough to
the magnet so the magnetic force affects the path of the ball.)
• NASA: What part of the model represents the launch pad, spacecraft, gravity, and the planet or
moon? Is the rope or magnet a better model of how spacecraft travel? (There are no physical guides
in space, but there are invisible forces. The magnetic field is a stand-in for gravity.)
• Career: Show kids the engineer profile featuring Victoria Garcia. As an aerospace engineer, she
uses virtual-reality tools to design effective spaces for astronauts living and working on a
spacecraft. Download the profile sheet for fun facts, discussion prompts, and extension ideas.
EXTEND THE CHALLENGE • Sharpest turn. Which team can consistently achieve the largest change in direction of the launch
path? Can anyone do better than a 90° bend? Which team can consistently hit the target?
• Go longer. Find a large, level area. Challenge teams to hit a target that is at least 30 centimeters
(1 foot) away. Next try 60 centimeters (2 feet) away.
CURRICULUM CONNECTIONS
Use Invisible Force to engage, explain, and extend student understanding of the following topics:
• Inverse square law. Gravity, electric charges, light, and sound are all governed by an inverse-square
relationship. Let’s use light as an example. As you move away from a light source, it gets dimmer.
But how much dimmer? If you double your distance from the source, the light you see will be ¼ as
bright as before, not ½ as bright (i.e., ½ squared = ¼). Because the intensity of the light varies as
the square of the distance, this relationship is called the inverse square law. With magnets, the
drop in the magnetic force is even more extreme—it changes as the cube of the distance!
• Newton’s 1st Law. Unless acted upon by an outside force, objects in motion tend to stay in motion,
and objects at rest tend to stay at rest. This law is key to understanding how spacecraft move and
navigate from Earth to their targets billions of miles away. Newton’s 1st law says that a spacecraft will
travel in a straight line and at the same speed forever unless an outside force acts on it. NASA often
uses gravity assists to supply the force necessary for changing a spacecraft’s direction and speed.
• Newton’s 2nd Law. As with the magnet, the closer the spacecraft passes to the planet or moon, the
more force will be exerted. As more force is applied to spacecraft, its acceleration—its speed and/
or direction—changes. There is a direct proportion between the two quantities.
Design Squad Nation™/© 2013 WGBH Educational Foundation.
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INVISIBLE FORCE
To make changes in a spacecraft‘s speed and direction, NASA often uses
gravity. To get what’s called a “gravity assist,” NASA passes a
spacecraft close to a planet or moon. The strong gravity changes the
spacecraft’s speed and direction.
CHALLENGE 1
Roll a ball down a ramp and use a ROPE to guide it to a target that’s set
off to the side.
MATERIALS
1. BRAINSTORM AND DESIGN
12.5 centimeters
[1 X 5 inches])
•30-centimeter (12-inch)
length of flexible rope
(e.g., clothesline)
•1 steel ball (e.g.,
60-millimeter [quarterinch] ball bearing)
•1 strong magnet
•1 target (e.g., “X” of
tape on the table or an
object to hit)
•Tape (any kind)
• How should you line up the rope with the end of the ramp so the
rope can guide the ball?
2. BUILD AND TEST
• Taping the ramp in place is very helpful for consistent results.
• You can change the ball’s speed by launching from different places
on the ramp.
3. EVALUATE, REDESIGN, AND RETEST
• What were the “wackiest” target positions that you could hit with
your ball?
•Paper cup (6- to 8-ounce)
•Strip of index card (2.5 x
WORDS TO USE
•magnetic field: The area
around a magnet where a
magnetic force can be
detected
•gravity assist: Using a
planet’s or moon’s gravity
to change a spacecraft’s
direction and speed
Design Squad Nation™/© 2013 WGBH Educational Foundation.
CHALLENGE 2
Roll a ball down a ramp and use a MAGNET to guide it to a target that’s
set off to the side.
1. BRAINSTORM AND DESIGN
• Where should you tape the magnet so the ball’s path curves?
2. BUILD AND TEST
Check out NASA’s
missions at nasa.gov
• F ind the right speed and distance from the magnet so the ball curves
over to the target.
3. EVALUATE, REDESIGN, AND RETEST
•H
ow extreme a turn can your ball make?
•H
ow many times in a row can you get the ball to hit the target?
NASA EXPLORES SPACE
Jupiter
Earth
Saturn
Uranus
Neptune
For most NASA missions, engineers pass
a spacecraft close to a planet or moon.
This lets them use the strong gravity to
change the spacecraft’s direction and
increase its speed. For example, NASA’s
Voyager mission got a gravity assist from
five planets to help it on its way!
Voyager
Visit the Design Squad Nation website at
pbskids.org/designsquad.
M A J OR F U N D I N G
PROJ E C T FUNDING
A DDITIONA L FU NDING
Design Squad Nation is produced by WGBH Boston. Major funding is provided by the National Science Foundation. Project funding is provided by Northrop Grumman Foundation and S.D. Bechtel, Jr. Foundation. Additional funding is
provided by United Engineering Foundation (ASCE, ASME, AlChE, IEEE, AIME). This Design Squad Nation material is based upon work supported by the National Science Foundation under Grant No. EEC-1129342. Any opinions,
findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. © 2013 WGBH Educational Foundation. Design
Squad and Design Squad Nation are trademarks of WGBH Educational Foundation. All rights reserved. All thrid party trademarks are the property of their respective owners. This NASA/Design Squad Nation
challenge was produced through the support of the National Aeronautics and Space Administration under Grant No. <NNX12AB47G> issued through the Science Mission Directorate.
DESIGN CHALLENGE PERFORMANCE
ASSESSMENT RUBRIC
Challenge name:
Names of team members:
Identifying the
problem(s) and
brainstorming
solutions
Showed a clear
understanding of the
problem(s) to solve.
Independently
brainstormed solutions.
Needed some teacher
direction to define the
problem(s) and brainstorm
possible solutions.
Needed lots of teacher
direction to define the
problem(s). Little if any
independent brainstorming.
Points:
Working as a
team member
Worked well together. All
team members
participated and stayed
on task.
Some team members w
ere
occasionally off task.
Most team members w
ere
often off task a
nd not
cooperating o
r participating
fully.
Points:
Using the design process
Team brainstormed many
design ideas and tested
and improved the design.
Final design complete or
nearly complete and
shows creative problem
solving.
Some team members w
ere
occasionally off task.
Team brainstormed few
design ideas and did little
testing or redesigning. Final
design lacks clear d
esign
idea(s).
Points:
Processing the Team gave a strong
presentation of its
science and
solution to the challenge
engineering
Team gave a basic
presentation of its s
olution
to the challenge a
nd
showed basic
understanding of the
science concepts and
design process.
Team gave a weak
Points:
presentation of its s
olution
to the challenge a
nd
showed little u
nderstanding
of the science concepts and
design process.
nd showed clear
a
understanding of the
science concepts and
design process.
Total Points:
Visit the Design Squad Nation website at
pbskids.org/designsquad.
M A J OR F U N D I N G
6
PROJ E C T FUNDING
A DDITIONA L FU NDING
Design Squad Nation is produced by WGBH Boston. Major funding is provided by the National Science Foundation. Project funding is provided by Northrop Grumman Foundation and S.D. Bechtel, Jr. Foundation. Additional funding is
provided by United Engineering Foundation (ASCE, ASME, AlChE, IEEE, AIME). This Design Squad Nation material is based upon work supported by the National Science Foundation under Grant No. EEC-1129342. Any opinions,
findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. © 2013 WGBH Educational Foundation. Design
Squad and Design Squad Nation are trademarks of WGBH Educational Foundation. All rights reserved. All thrid party trademarks are the property of their respective owners. This NASA/Design Squad Nation
challenge was produced through the support of the National Aeronautics and Space Administration under Grant No. <NNX12AB47G> issued through the Science Mission Directorate.
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